This invention relates to a circuit board testing apparatus and method for testing such electric state as continuity, open-circuit, short-circuit, of a number of wirings formed on a circuit board.
It should be appreciated that this invention is applicable to testing of electric state of wirings formed on any of a variety of circuit boards or substrates such as printed circuit boards, flexible circuit boards, multi-layer circuit boards, glass substrates for use in liquid crystal display or plasma display panels, and film carriers for use in semiconductor packages, and that the term “circuit boards” as used in this specification may be any of these variety of boards.
Circuit boards are formed with a wiring pattern by a number of wirings thereon. There have been proposed a number of testing apparatus to test whether the wiring pattern has been formed as designed. Recent trend of making small-sized and light-weighted electronic devices necessitates arranging a wiring pattern in a complex manner in a small area. Thus, it is difficult to test open-circuits and short-circuits of wirings by direct contact of probes with the circuit board. Therefore, there has been proposed a contactless testing apparatus to test electric state of a wiring pattern such as open circuit without causing direct contact of probes with a minute conductive pad.
For example, Japanese Patent No. 3080158 discloses this type of apparatus which is adapted for testing an open or short-circuit of a wiring formed on a circuit board. In the apparatus, specifically, an electromagnetic wave is irradiated onto a pad connected to each wiring of a circuit pattern formed on a circuit board to thereby cause discharge of electrons from the pad owing to photoelectric effect. The open or short-circuit of the wiring is tested based on an electric current which is caused by discharged electrons to run into a ground (GND) plane or external metallic plate capacitively coupled.
Japanese Unexamined Patent Publication No. 8-278342 discloses a printed board testing apparatus which detects electrons discharged in a space owing to photoelectric effect to test continuity or open-circuit of wirings of the printed board. Specifically, an electric charge sensor and an electromagnetic wave generator are movably provided above the printed board with a specified gap or distance therebetween. The electric charge sensor and the electromagnetic wave generator are relatively moved over the printed board to scan discharged electrons. The electric state of wirings are judged based on detected changes of the electric current.
The aforementioned conventional arts are suffered from the following drawbacks. In the conventional art, an electromagnetic wave is merely irradiated onto a pad or wirings. Electrons which are discharged owing to photoelectric effect upon irradiation are returned to the pad and wirings, or dispersed in the space without being utilized for the testing.
Further, discharged electrons form a spatial charge region, and lower the electron discharging efficiency of the photoelectric effect. Even if electrons are discharged instantaneously owing to photoelectric effect, accordingly, current flowing in the ground plate or external metallic plate cannot be measured with reliability. Thus, it is difficult to accomplish stable and precise test efficiency.
In the apparatus of Japanese Unexamined Patent Publication No. 8-278342, further, the electric charge sensor and the electromagnetic wave generator are moved relatively to the printed board to scan discharged electrons, which consequently increases the size of the apparatus. It will be seen that in the case of producing a vacuumed space between the printed board and the electric charge sensor and the electromagnetic wave generator, a larger-sized vacuuming unit is required.